Energy intake and dietary macronutrient composition is the most difficult part of the energy balance equation to measure. In a natural history study of factors which predict food intake and using an inpatient computerized vending machine system, food intake has been measured over 250 individuals. In individuals undergoing the study more than once the intra-class correlation coefficient is very high (r=0.9) indicating that these studies although performed in an inpatient setting are very reproducible. This study has demonstrated that higher respiratory quotient, the ratio of carbohydrate oxidation to lipid oxidation which predicts weight gain, also predicts food intake. The component of respiratory quotient which best predicted food intake was higher carbohydrate oxidation indicating a role for carbohydrate (likely glycogen) balance in the regulation of short term food intake. We have also demonstrated that one of the most important determinants of food intake is fat free mass. There is a strong positive association between fat free mass (adjusted for height) and food intake, which persists even when adjusted for fat mass. Fat free mass is the major determinant of energy expenditure, and 24 hour energy expenditure is also strongly associated with energy intake. Furthermore, even after adjustment for fat free mass, the residual of energy expenditure was still associated with energy intake. In models that include measures of body composition, 24 hour energy expenditure and respiratory quotient, only energy expenditure and respiratory quotient remain associated with food intake. This indicates homeostatic sensing of energy needs driven largely by metabolic rate and substrate oxidation. .We have developed a protocol investigating if increases in energy expenditure (as induced by cold and one medication) will lead to increased food intake. We have also found that calories from soda intake as measured while individuals ate from our vending machines, predicted future weight gain indicating that sweetened beverages are contributing to our current obesity epidemic. We have also demonstrated that perseveration as measured on the Wisconsin Card Sort performance task was positively associated with ad libitum food intake, and restraint as measured on the three factor eating questionnaire was negatively associated with ad libitum food intake. However, there was an interaction between perseveration and restraint such that participants with high perseveration and high restraint ate less than the other groups. To develop methods to measure free living food intake, we have ongoing studies which are validating chewing and swallowing monitors using our automated vending machines. Our initial pilot study of ten individuals demonstrated that eating events can be captured on these monitors and that overall eating events do correlate with calorie intake, but we are continuing to follow-up on these preliminary results with an upgraded chewing monitor to better capture eating episodes. We are continuing our study investigating stable isotopes as biomarkers for dietary patterns. Individuals (n=40) will consume diets over 3 months which vary by meat, soda and fish content and plasma, hair, and adipose tissue samples will be analyzed for changes in the stable isotopes C13 and N15. We will also investigate how these dietary patterns affect behavioral and performance testing, glucose tolerance and energy expenditure. We have currently completed 24 volunteers in this intensive study which requires a 14 week stay on our inpatient unit. Preliminary analyses indicate that we can differentiate that N15 enrichment does identify individuals who are consuming fish over the 13 weeks study period.